It sounds too good to be true, but scientists are looking into whether they can harness fat-burning brown fat for weight loss.

What if fat weren’t the enemy after all and could actually help you lose weight?

Earlier this year, scientists at the Dana-Farber Cancer Institute published a study showing that injecting inactive mice with a natural hormone from muscle cells gives some of the benefits of exercise by turning regular fat into an energy-burning form of fat—without ever breaking a sweat.

The potential for treating weight-related ailments, from diabetes to obesity, is enormous, and the scientists have already licensed their findings for drug development.

According to the study, the hormone “causes a significant increase in total body energy expenditure and resistance to obesity-linked insulin resistance.” And it gives “some of the most important benefits of exercise and muscle activity.”

So does this mean that one day you can skip the gym altogether and just have an “exercise injection”?

The Skinny on Brown Fat

First, let’s take a look at what this energy-burning fat actually is. Fat, or adipose tissue, comes in two main types: white fat and brown fat. White fat is the most common. As the name suggests, this type of fat is white in color, due to its main component, triglycerides, which are also white. This fat forms globules and stores energy, which the body uses up during exercise or periods of fasting.

White fat is what we generally talk about “losing” through diet and exercise, although we never actually lose fat cells through either of these actions. Instead, fat cells shrink when we lose weight and expand when we gain it.

Brown fat, on the other hand, differs from white fat in two ways. First, it doesn’t accumulate in energy-storing globules. Second, it is brown in color due to an overabundance of mitochondria, a structure that exists in nearly every cell in the body and turns food into energy through the production of adenosine triphosphate (ATP). But brown fat mitochondria work a little differently than the mitochondria in other cells. Instead of producing most of their energy as ATP, they make energy in the form of heat.

So what does all that mean? Essentially, white fat stores energy and brown fat burns it.

Scientists have known for decades that rodents and other mammals have brown fat, which likely evolved as a source of heat during hibernation. It’s also well known that human infants have a significant amount of brown fat, also likely to help keep them warm. But it wasn’t until 2009 that scientists discovered that small amounts of brown fat are still active in adult humans, mainly in the neck and along the collarbone and spine.

Ever since the discovery of brown fat, scientists have tried to figure out how to use it for weight loss, notes George Bray, M.D., professor of medicine at the Pennington Biomedical Research Center at Louisiana Statue University, who was not involved in the research. “[Brown fat] has been around for quite awhile, and so far it hasn’t netted any therapeutic results,” he says. “But maybe this new finding will help do something that no one’s been able to do before.”

Some researchers believe that our bodies may be able to transform white fat into brown fat, forming what they call “beige fat.” Theoretically, this could mean that the key in using brown fat for weight loss is to understand the mechanisms behind this browning effect and then manipulate them.

The Dana-Farber study suggests one possible mechanism that could produce beige fat: a new hormone, which they discovered, that carries chemical messages from muscle tissue to fat tissue during exercise. The hormone was named irisin for Iris, the Greek messenger goddess. A protein called PGC1-alpha responsible for the regulation of genes related to metabolism also controls the activity of irisin.

The scientists found that irisin increases in both humans and mice after exercise. And when they injected irisin into the white fat in inactive mice, it appeared to turn on genes that may be responsible for browning the fat, or creating beige fat. The mice that were injected with the hormone also lost weight and were better at controlling blood sugar levels, compared to mice that were not injected.

Interestingly, the irisin found in mice is identical to the hormone in humans. This is unusual. Other metabolic hormones that are studied in mice, such as insulin (which regulates sugar in the bloodstream) and leptin (which helps curb hunger), are only 85 percent and 83 percent identical between mice and humans, respectively. The fact that irisin is precisely the same in humans as in mice means that it may also function the same way. This could mean that an irisin injection for humans would give the same benefits that the mice showed in the study.

But don’t get your hopes up just yet. Irisin might not be the miracle hormone that it appears to be, cautions David Levitsky, Ph.D., a professor of nutrition and psychology at Cornell University. “There are hundreds of hormones and chemicals in the body that are released during exercise that, if you inject them in animals, will cause them to stop eating or that will turn on the sympathetic nervous system that turns brown fat on,” he says. “Just finding that a substance is released through exercise and causes an inhibition of appetite doesn’t mean that is how it normally works.”

There could still be benefits from the hormone, he adds, but irisin likely acts on other receptors in the body in ways that aren’t yet clear. The next step is to find out how much irisin is released naturally during exercise and whether that amount affects the body the same way as the amount injected, as well as to better understand the other ways irisin works in the body.

An Exercise Injection?

Could the research eventually lead to an injection that replaces exercise? Not so fast. Even if irisin injections work the same way in humans as they do in mice, the experiments so far suggest that the hormone does not make muscles stronger. For fit muscles and a healthy cardiovascular system, you would still need to hit the gym.

Still, if such a drug were developed, researchers believe that it may help treat a number of metabolic diseases, including diabetes, insulin resistance and obesity, as well as neurodegenerative diseases including Parkinson’s. And according to a press release from Dana-Farber, the potential drug could also help treat and prevent certain cancers that are associated with obesity and inactivity.

“The implications are down the road a bit,” says Dr. Bray. “We get interesting experimental findings and they may or may not translate into anything therapeutically relevant. But I think it is a provocative and interesting idea. We know muscle is involved in improving responses to diabetes and [irisin] might be that important link that’s involved. It certainly is worth pursuing and it will be actively pursued.”

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